It's not getting enough light.

Maybe it's rootbound and needs repotting?

what type of tomato plant is it?

If it's a determinate type, it'll flower, make toms, then stop. That's all she wrote.

I don't know what type it is.

I can't believe it's not getting enough sun, I had it in the same window my pepper plant was in, and it's actually putting out new growth and new blooms right now.

Like I said, this one came out of one of the pots my gf & her mother have, and theirs are a couple years old. But they are also in front of bigger windows so maybe that helps.

Could be a lot of things. Too little light, too close to the cold of a window, needs fertilizer, too much fertilizer, too much water, too little water, insect pests, fungal trouble, etc, etc. Do you have a photo?

And we'll assume you used a fungicide, not an herbicide on the plant. If you used an herbicide, you're lucky to have any plant left alive at all!

garwv, of course it grew like crazy outside in the summer.

A greenhouse would help. Tomato plants generally don't do well when grown indoors. The air is too dry and there's not enough humidity or light to support them over a long period of time. Tomatoes need 6 hrs of full sun to make fruit and the winter sun is very weak. The window deflects and weakens the light cast upon the plants.

I've brought small sized tomato plants in for the winter and they do fine for awhile but they do go downhill and eventually die.

You could keep babying the plant and maybe get a cutting from it in early spring.

Btw, I do winter over peppers under lights in an unheated well lit room. They struggle but they still produce some peppers.